Thermoplastic materials often contain high levels of colorants, reinforcing agents, and other constituents to impart certain desired properties to the finished articles. These materials are typically monolayered and are extruded into various shapes by well-known methods. Although the extruded materials exhibit good overall properties when lower levels of colorants or reinforcing agents (e.g., less than about 10% by weight for glass; less than about 0.5% nonvolatile weight for some colorants) are incorporated, several serious problems arise when higher levels of the colorants and/or reinforcing agents are added. For instance, the colorants, especially the organic dyes, are susceptible to plateout, which is a migration of the colorants from the interior of the extruded material to the surface of the material during and following the extrusion process. These colorants may then become deposited on the hot chrome polishing roll and other metal surfaces of the extruder, resulting in scratches and excessive wear on the roll surface, as well as causing excessive smoking in the vicinity of the hot extruder. Moreover, the deposited colorants may contaminate other materials which may be subsequently extruded. Furthermore, the colorants (as well as certain stabilizers and processing aids) may bleed, i.e., migrate to the surface, and react with subsequently applied coatings or inks, thereby resulting in undesirable color changes, printing smudges, and possible delamination of the subsequently applied materials. Moreover, the propensity for colorants to migrate to the surface of the material necessitates high loading levels of colorants to compensate for optical deficiencies which would result from reduced colorant levels in the interior of the material. Since colorants are generally expensive materials, the cost for incorporating such high levels is high.
When reinforcing agents, especially glass filaments, are included at levels of above about 10% by weight in the composition being extruded, they tend to break through the surface of the monolayer material and cause abrasions die lines, grooves, and other imperfections which may eventually result in mechanical failure of the material. Furthermore, the reinforcing agents frequently become caught on the surface of the extruder die and in the melt pipes, thereby resulting in further wear and possible breakdown of the extruder.